With a Third Generation Partnership Project (3GPP) Time Division Duplex (TDD) system, time is partitioned into transmission time intervals (TTIs) that are subdivided into frames, which are further subdivided into timeslots. A TTI is defined as one or more radio frames. Specifically, a radio frame is 10 ms; and a TTI may be 10, 20, 40 or 80 ms. The low chip rate TDD divides each frame into two subframes. The subframes are then divided into timeslots. A Coded Composite Transport Channel (CCTrCH) comprises one or more Transport Channels (TrCHs). A CCTrCH is mapped into a collection of one or more sets of timeslots and codes.
When the maximum data size of a CCTrCH is transmitted, all allocated codes and timeslots are used in the TTI. The actual number of codes and timeslots that are transmitted during a TTI are signaled to the receiver via a Transport Format Combination Index (TFCI). Codes and timeslots are allocated according to a set of rules known to both the transmitter and receiver, so once the number of codes and timeslots are known to the receiver by decoding the TFCI, it also knows which codes were transmitted in each timeslot.
A 3GPP TDD system includes support for discontinuous transmission (DTX) of radio frames when the total bit rate of a CCTrCH is less than the total bit rate of the codes and timeslots allocated to the CCTrCH within a TTI. The coding and multiplexing function in a TDD transmitter maps data onto codes and timeslots.
DTX is applied separately to each CCTrCH. When a CCTrCH is in DTX, some or all of the codes and timeslots allocated to the CCTrCH are not transmitted. DTX falls into two categories referred to as partial DTX and full DTX. During partial DTX, a CCTrCH is active but less than the maximum number of codes and timeslots are filled with data, and some codes and timeslots are not transmitted within the TTI. During full DTX, no data is provided to a CCTrCH by upper protocol layers and there is no data at all to transmit within a TTI.
During non-DTX operation and partial DTX, the first timeslot allocated to a CCTrCH in each frame will contain at least one code to transmit the TFCI. The rules for determining which code contains the TFCI are known to both the transmitter and receiver, so the receiver always knows exactly where to find the TFCI. The CCTrCH may have additional transmitted codes in the same timeslot, additional transmitted codes in subsequent timeslots in the same frame, or additional transmitted codes in subsequent timeslots in subsequent frames of the TTI. The transmitted codes and timeslots may change from one TTI to the next; however, the first timeslot allocated to a CCTrCH in each frame will always contain at least one code to transmit the TFCI. A CCTrCH may comprise multiple TrCHs that have different TTIs. In that case, the transmitted codes may change during each interval equal to the shortest TTI among the TTIs for all TrCHs in the CCTrCH. Throughout this document, references to the TTI will mean the shortest TTI among all the TrCHs in the CCTrCH.
Referring to FIG. 1, an example CCTrCH is shown having codes allocated in timeslots 2, 3, 4, and 5 of a frame and a TTI of 20 ms (that spans two frames). The same codes are allocated to the CCTrCH in both frames of the TTI; however, due to partial DTX they are not all transmitted. In FIG. 1, X indicates allocated codes that are transmitted and Y indicates allocated codes that are not transmitted. In both frames, code 1 in timeslot 2 is transmitted and contains the TFCI. Once the receiver decodes the TFCI in the first frame, it knows which codes and timeslots are transmitted in both frames. It should be noted that since the location of the TFCI is configurable, it could optionally be sent in more timeslots of a particular CCTrCH or within all timeslots of the CCTrCH.
During full DTX, periodic special bursts (SBs) are transmitted and identified by a 0-valued TFCI in the first code of the first timeslot allocated to the CCTrCH. The SB indicates the start of full DTX. Subsequent SBs are transmitted every Special Burst Scheduling Parameter (SBSP) frame. The subsequent SBs provide a mechanism for the receiver to determine that the CCTrCH is still active, and prevent the receiver from declaring out-of-sync. Full DTX ends when upper protocol layers provide data.